1. Field of the Invention
The present invention relates to a method of flattening a sapphire substrate obtained by slicing an ingot.
2. Description of the Related Art
The surface of a wafer obtained by slicing an ingot with a wire saw has fine undulation (irregularities) and strain. To remove such undulation and strain, both sides of the wafer are ground and polished to be flattened. For example, a wafer flattening method described in Japanese Patent Laid-Open No. 2011-151099 includes the steps of measuring the undulation of a wafer, applying a liquid resin to a film to form a resin layer on the film, attaching one side surface of the wafer to the resin layer, selectively applying ultrasonic waves to the wafer according to the undulation measured above to thereby partially reduce the viscosity of the liquid resin and restore the undulation of the wafer, curing the liquid resin, and grinding the other side surface of the wafer to remove the undulation on the other side surface of the wafer, thus sufficiently removing the undulation on the other side surface of the wafer irrespective of the undulation on one side surface of the wafer.
Further, a wafer flattening method described in Japanese Patent Laid-Open No. 2011-249652 includes the steps of holding one side surface of a wafer on a horizontal holding surface under suction to thereby correct the undulation on one side surface of the wafer and make this surface horizontal, grinding the other side surface of the wafer to remove the strain on the other side surface, holding the other side surface of the wafer on the horizontal holding surface under suction to thereby correct the undulation on the other side surface of the wafer and make this surface horizontal, grinding one side surface of the wafer to remove the strain on one side surface, whereby similar grinding strains are given to one side surface and the other side surface of the wafer, and finally flattening both side surfaces of the wafer, so that a warp or bow due to the strain in the wafer can be effectively removed to achieve high flatness.